Week 8 -Wednesday What did we talk about last time? StdDraw - - PowerPoint PPT Presentation

Week 8 -Wednesday

 What did we talk about last time?  StdDraw practice

 We have written lots of code so far  It has all been inside of the main() method  What about a big program?  The main() method is going to get really long and hard to

read

 Sometimes you need to do similar things several times:

• Read some input and check that it falls within certain values
• Draw a green hexagon at several different locations
• Find the square root of several different numbers

 Methods allow you to package up some code to run over and

• ver

 Methods usually take some input (like numbers or Strings)

so that they can be customized

 Methods often give back an answer (like the square root of a

number)

 Also called functions in some other languages

 More modular programming

• Break a program into separate tasks
• Each task could be assigned to a different programmer

 Code reusability

• Use code over and over
• Even from other programs (like Math.sqrt())
• Less code duplication

 Improved readability

• Each method can do a few, clear tasks

public static type name(type arg1,…,type argn) { statement1; statement2; … statementm; } Required syntax Type of answer Name of last argument Name of 1st argument Name of method Type of 1st argument Type of last argument Code done by method

 Given two integers, find the smaller: public static int min(int a, int b) { if( a < b ) { return a; } else { return b; } }

 static methods are methods that are not connected to a

particular object

 They are just supposed to perform some simple task  We are going to focus on static methods until next week  For now, just taken it as a given that the definition of every

method will include the static keyword

 It is possible to divide methods into two types:

• Value returning methods
• Void methods

 Value returning methods give an answer:

int small = min(x, y);

 Void methods are declared with void as their return type  Void methods just do something  If you try to save the value they give back, there will be a compiler

error

public static void help(int times) { for(int i = 0; i < times; ++i){ System.out.println("Help!"); } }

 Like most code in Java, the code inside of a method executes

line by line

 Of course, you are allowed to put if statements and loops

inside methods

 You can also put in return statements  A method will stop executing and jump back to wherever it

was called from when it hits a return

 The return statement is where you put the value that will be

given back to the caller

 Defining a method is only half the game  You have to call methods to use them  Calling a method means giving a method the parameters (or

arguments) it needs and then waiting for its answer

 By now, you have done many method calls

• System.out.println()
• Math.sqrt()

 You can call your own methods the same way

 Proper syntax for calling a static method gives first the name

• f the class that the method is in, a dot, the name of the

method, then the arguments

 If the method is in the same class as the code calling it, you

can leave off the Class. part

 If it is a value returning method, you can store that value into

a variable of the right type

Class.name(arg1, arg2, arg3);

 If a static method is in another class, you have to put the name of

the class first, followed by a dot

 We've seen a few of these already:  By convention, class names in Java always start with a capital

letter

double length = Math.sin(angle); double value = Math.random(); double[] samples = StdAudio.read(file);

 Variables from outside of the method don't exist unless

they've been passed in as parameters

 No matter how complex a program is, inside this method,

• nly x, y, and z variables exist

public static int add(int x, int y){ int z = x + y; return z; }

 A magical process called binding happens which copies the

values from the calling code into the parameters of the method

 The calling code can use variables with the same names, with

different names, or even just literals

 The method does not change the values of the variables in the

• riginal code

 Remember, it only has copies of the values

 No connection between the two different x's and y's

public static int add(int x, int y){ int z = x + y; //5 + 10 return z; } int a = 10; int x = 3;

int y = add( 5, a ); //y contains 15 now

• 1. Start a method with the keywords public static
• 2. Next comes the return type
• 3. Then the name of the method
• 4. Then, in parentheses, the arguments you want to give to the

method

• 5. Inside braces, put the body of the method
• 6. Include a return statement that gives back a value if

needed

 A palindrome is a word or phrase spelled the same forwards

and backwards:

• "bob"
• "radar"
• "satanoscillatemymetallicsonatas"

 Write a method with the following header that returns true

if a String is a palindrome and false otherwise

public static boolean isPalindrome(String phrase) {

 Keep reading Chapter 8  More method practice

 Keep reading Chapter 8 of the textbook  Keep working on Project 3

• Due next Friday